The drilling of horizontal or other laterally extending well bores to re-enter existing wells is commonly employed to extract additional hydrocarbons from a field in which production has decreased. Horizontal or other lateral drilling operations, which for convenience will be referred to simply as lateral drilling or lateral wells, provide the benefits of improving the depletion and increasing the drainage area of the reservoir, reducing water and gas coning, and improving production rates. Short-radius wells are preferred in many installations.
The initial stage of the drilling of a lateral well, also referred to as “sidetracking”, requires that the drill bit pass through an opening in the wall of the casing and penetrate the adjacent formation. As a first step in the sidetrack drilling operation, a section of the well is filled with a cement composition slurry and is allowed to harden, or cure, to form a plug between the casing and the formation.
The ability to successfully initiate a lateral well at the desired position in a vertical well bore is dependent upon the drill bit being deflected, with minimum penetration of the cured cement plug that has been previously pumped into place, and then biting into the formation rock at the desired angle. If the cement is not sufficiently hard, the drill bit will not be deflected from the original well bore, but rather will re-enter the original well bore and will not penetrate the rock formation at the desired angle, if at all. It is not uncommon for several attempts to be required before the sidetrack bore is successfully completed. Thus, a cement of a hardness that is the same as, or as close as possible to that of the formation rock would be highly desirable to improve the success rate and efficiency of initiating lateral well bores.
As used herein, the terms “cement” and “well cement” will be understood to have their art-recognized meanings of a composition that is employed in various aspects of well drilling and completion operations and in which an hydraulic cement, such as Portland cement or other mineral-derived compounds containing lime, alumina and silica constitute one of the ingredients.
A particularly useful measure of a well cement's hardness in the context of lateral well drilling is the rate of penetration (ROP) of a drill bit, typically measured in feet per hour. A method and apparatus for conducting this measurement is described in the Society of Petroleum Engineer's publication identified as SPE 37821, March 1997, authored by S. S. Jennings, the disclosure of which is incorporated herein by reference. Core samples taken from the well bore or a comparable geological formation are also subjected to testing to obtain an ROP value for the purpose of matching, as nearly as possible, the ROP value for a given cured cement composition.
It has been found that increasing compressive strength by reducing water content leads to a cement with low impact strength and low resistance to fracture propagation.
It is known that the addition of ceramic proppants of the type used in formation fracturing will provide a desired increase in the hardness of a well cement. The use of larger ceramic proppants produced lower ROP values, i.e., slower drilling rates. Data reported included the reduction from 6.40 ft/hr with inert ceramic fracturing proppants. A further reduction in the ROP values for well cements to a value that is preferably equal to, or even more preferably, lower than the ROP value of the formation rock to be drilled would be advantageous for use in the art. However, the addition of larger particles and aggregate materials detrimentally affect the cement pumping and handling equipment by blocking valves.
It is therefore an object of the invention to provide a cement composition for use in sidetracking operations for lateral well drilling that, when cured, has a high compressive strength and increased hardness as compared to cements known in the art.
Another object of the invention is to provide a novel combination of additives for formulating hydraulic cement compositions that will permit the user to minimize the difference in the ROP of the well cement plug and the adjacent formation that is to be bored laterally
It is another object of the present invention to provide a method for identifying an optimum well cementing formulation for use in initiating a lateral well drilling operation to facilitate penetration of a specified rock formation.